Over the past decade, the use of pigmented synthetic fibers and thin films has grown apace despite the known proclivity of such fibers and thin films to change color long before they lose their integrity. A large proportion of such fibers and thin films are formed from stabilized polyolefins, and other normally solid predominantly ethylene-and propylene-containing copolymers (together referred to hereafter as "PO" for brevity), and are used for a host of different applications.
Retention of colors red, blue, yellow, and orange, and shades of these colors, derived from azo, disazo and phthalocyanine pigments, in PO fibers and thin films exposed to sunlight over their useful life, is of great practical value in clothing, drapes and other articles made from woven or non-woven fabrics of the pigmented PO fibers. But Red 144, Orange 34 and Yellow 93 are known prodegradants; and, PO pigmented with these pigments, not only degrades the PO but it loses its pigmentation due to chemical degradation of the pigment even before the PO itself is degraded past the point where the articles serve their intended use.
This invention is directed to a particular combination of specific hindered piperidyl-based (PDYL) compounds, known as hindered amine light stabilizers (HALS), having plural polysubstituted piperidyl rings in a molecule, which HALS is combined with a 3,5-disubstituted-4-hydroxybenzoate (generically referred to as "3,5-DHBZ"), provides surprisingly good stabilization, provided the pigmented PO is either in the form of fibers less than 50 microns (.mu.) thick, or, in the form of film less than 50.mu. thick, and further providing the pigment is red, blue or green, and shades thereof, in particular classes, namely azo, disazo and phthalocyanine pigments. By "polysubstituted" piperidyl ring, I refer to a ring in which the N-adjacent C.sup.3 and C.sup.5 atoms are either disubstituted or substituted with at least one cyclic substituent.
Stabilizing fibers used in fabrics against degradation is a particularly difficult problem because such fibers are so thin (less than 50.mu. in diameter. See "The Photooxidative Degradation of Polypropylene. Part II. Photostabilization Mechanisms" by Carlsson D. J. and Wiles, D. M. J. Macromol. Sci.--Rev. Macromol. Chem., C14(2), 155-192 (1976)). Light striking the surface of a fiber has a very short distance to travel since additives (absorbers) used at a conventional level, say 0.1 phr (parts per hundred of resin), cannot function to protect the polymer which degrades because of its inherent absorptivity. The well-founded expectation is that because most films and fibers have a cross-section less than 50.mu. in thickness, adequate protection by UV absorption alone is impossible (see Text. Res.J. 39 243 (1969, J. E. Bonkowski). Further, the sensitivity of PP homopolymer articles to surface photooxidation implies that the UV absorption alone may be of only limited use even in the protection of thick moldings because of the massive surface damage which must result in the presence of the normal amount of a uniformly distributed UV absorber (see Macromolecules 4 174 (1971), Carlsson and Wiles). Both of these conclusions are factual confirmation of the effects of Beer's law.
The effect of degradation as a function of depth, measured from the surface exposed to UV radiation, on stabilized polypropylene, was investigated by K. W. Leu who presented his findings at the Plastics Institute of Australia Inc. 1974 Residential Technical Seminar. He exposed only one side of PP plaques to 150 Kly (kilolangleys) in an outdoor weathering station in South Africa, then sectioned thin slices (microtomed) of the plaques, starting at the exposed surface, dissolved them, and measured the viscosity of the solution.
The results were presented in a graph reproduced herein as FIG. 1, in which the dashed line A is for unexposed PP, which of course shows maximum viscosity and no degradation, therefore is parallel to the abcissa along which thickness in millimeters is plotted. The line A is drawn along the abcissa for a total thickness of 78.7 mils (2.0 mm).
Line B, the lower parallel line, is for plaques stabilized with 0.3% of a piperidyl-based HALS. The ordinate indicating some degradation, measured by viscosity as before, is about 1.7 viscosity units, quantified by the height of the line identified by the legend S.sub.1.
Curve C plots viscosity as a function of thickness for plaques stabilized with 0.3% of a UV absorber (UVA), for example, a benzophenone or benzoate. This curve starts near the intersection of the axes indicating the viscosity is zero at the surface of the first section, then rises steeply and begins to plateau at about 2.2 units, at a depth of about 0.35 mm (13.8 mils). The ordinate marked S.sub.2 indicates that the improvement in stability provided by the 0.3% UVA is about 0.5 units (the difference between 2.2 and 1.7). Referring further to curve C, but at a depth of 50 microns (0.005 mm), it is seen that the ordinate identified as S.sub.3 measures up to about 0.1 viscosity unit. This represents a minimal level of stabilization provided by 0.3% UVA alone in the first section (a zone 50.mu. thick), taken near the surface.
Curve D presents the degradation of plaques stabilized with a combination of 0.15% UVA+0.3% HALS. Beyond a depth of about 0.35 mm it is seen that the stabilization provided is only slightly better than that provided by 0.3% UVA by itself, indicating very little contribution from the HALS. If the effect of the HALS, as depicted by line B, was additive, curve D would show much greater stabilization (be much higher viscosity) at a depth greater than about 0.35 mm.
Referring further to curve D at the ordinate identified by the letter S.sub.4 at a depth of 50 microns, the height of this ordinate (about 1.8 viscosity units) represents the level of stabilization provided by 0.3% HALS+0.15% UVA. Thus it is seen that at a depth of 50.mu. (0.005 mm), the illustrated combination of HALS and UVA provides no more than the additive stabilization one would expect them to provide.
Curve E relates to use of an antioxidant and is irrelevant to the subject matter at hand.
Understanding the foregoing, there was every reason to believe that a combination of a low level of piperidyl-based HALS, about 0.3% or less, in combination with a low level of a UVA, about 0.3% or less, would provide no more than a marginally greater stabilization than the HALS itself, within a 50 micron zone. Certainly there was nothing to suggest that the addition of a phthalocyanine or an azo pigment, known to be a prodegradant with piperidyl-based HALS in several synthetic resinous systems, to the combination of piperidyl-based HALS and benzoate (but not benzophenone) stabilizer, would reasonably be likely to provide a sudden boost in stabilization, rather than a merely additive effect.
As one would expect, loss of color is a particularly acute problem for PO fibers irrespective of the pigment used. It so happens that the problem is the most serious with phthalocyanine, azo and disazo pigments (the latter two being together referred to herein as "azo pigments") which in most other respects are ideal pigments for polyethylene (PE) and polypropylene (PP). Typical of such disazo pigments is Red 144 (common name), and the monoarylide or diarylide azo pigments Orange 34 and Yellow 93; typical of phthalocyanine pigments are (a) Blue 15, Blue 16 and Blue 29 with shades of blue available, for example, as Blue 15:1 through Blue 15:6; and, (b) Green 7, Green 36-3Y and Green 36-6Y. All are used in huge (tons) quantities to pigment PO fibers. For unknown reasons, however, we find that the orange and yellow colors in PO are not stabilized by the PDYL+3,5-DHBZ combination.
Phthalocyanine pigments have a generally flat tetra benzo tetra azo porphin structure. The pigments are usually made by the reaction of a phthalic acid derivative at a temperature of about 190.degree. C. with a source of nitrogen such as urea and a metal or metal salt. Molybdates, vanadates, and certain compounds of titanium have been found to be useful catalysts for this condensation reaction.
Red 144 (Registry No. 5280-78-4) is a disazo condensation pigment. The yellow pigments are either monoarylides or diarylides. The orange pigments are azo based and may or may not include a benzimidazolone structure.
To combat the problem of color fading, a better solution is constantly being sought to decelerate color loss which is at least as important as stabilization of the fibers of the PO. With particular respect to articles of Red 144-pigmented PP fibers which are in high demand, the use of Red 144 both hastens their degradation when exposed to sunlight, and degrades their physical properties over time. The combined effects subject the articles to a two-pronged attack on their longevity in normal use, thus vitiating their marketability.
In particular, fabrics made with Blue 15- and Red 144-pigmented PP fibers and stabilized with piperidyl-based HALS, are especially popular in automobiles, boats, outdoor clothing and other such uses where the fibers degrade at such an unacceptably high rate upon exposure to sunlight, that red articles are soon transformed into non-uniformly colored articles sporting a wide spectrum of unwanted shades of pink and orange; articles of blue fibers turn milky. The obvious way to cope with this color degradation problem is to use far more pigment than is required to provide the desired color, so that upon suffering the expected color degradation, the coloration of the remaining non-degraded pigment will maintain acceptable, if not the original, color. Except that `loading up` the HALS-stabilized fibers with more phthalocyanine pigment than necessary may lead to "bronzing"; loading up with Red 144 or other azo pigment generates a high proclivity towards reaction of pigment with the PDYL, and with other additives such as antioxidants ("AO"s) and anti-ozonants used to provide melt-stability to the PP. Further, increasing the concentration of pigment above about 1 phr may produce "blooming" of the pigment long before degradation of either the pigment or the fiber.
This invention particularly relates to the stabilization of fibers of PO, specifically of PE and PP fibers colored with phthalocyanine and azo pigments which provide red, blue and green colors, and shades thereof; more particularly, it does not relate to those azo pigments which provide either an orange, or yellow color.
It is known that several stabilizers, particularly the piperidyl-based HALS, by themselves, provide excellent stabilization of PO to heat, light and ultraviolet radiation, and, some hindered phenol stabilizers are antioxidants which provide both excellent thermoxidative stabilization, and light stabilization of PO, but such stabilization does not extend to that of color in phthalocyanine or azo-pigmented PO.
In U.S. Pat. No. 4,035,323, Mathis discloses that specific piperidyl compounds in combination with 3,5-DHBZ provide protection against sunlight. The specific compounds were tested in unpigmented 5 mil thick films with the assumption that if pigmented, the stabilization effect of the combination will endure, irrespective of the type of pigment used. This assumption was unfounded, at least with respect to azo and phthalocyanine pigments, as is evidenced by data presented hereafter.
The foregoing assumption as to the inertness of pigments, irrespective of their chemical structure, was dispelled relative to synergistic combinations of primary and secondary antioxidants in low concentration of each component (0.05% w/w each) in a study titled "Photo-Stabilising Action of a p-Hydroxybenzoate light Stabiliser in Polyolefins: Part III--Antioxidant Behaviour and Additive/Pigment Interactions in High Density Polyethylene" by Allen, Norman S. et al Polymer Degradation and Stability 10 (1985) 1-13. The same study pointed out that antagonism (not synergism) was exhibited at higher concentrations of the antioxidants. The study showed that a particular benzoate (Cyasorb UV 2908) with copper phthalocyanine (1% w/w) was more effective than when a synergistic combination of antioxidants (Irganox 1076 and Weston 618) were added. Though they tested a combination of Tinuvin 622 1-(2-hydroxyethyl)2,2,6,6-tetramethyl-4-hydroxypiperidine, succinic acid polymer; (a PDYL) and 3,5-DHBZ with thick film pigmented with titanium dioxide, they did not test Tinuvin 622 with an azo or phthalocyanine pigment in their films. The combination of Cyasorb 2908 (3,5-DHBZ)+Irganox 1076 and Tinuvin 622 (oligomeric HALS with several piperidyl groups) showed embrittlement at 2935 hr. The combination of Cyasorb 2908 (3,5-DHBZ)+Irganox 1076 and Tinuvin 770 N,N'-bis(2,2,6,6-tetramethyl-4-pieridinyl)hexamethylenediamine,2,4,6-trich loro-1,3,5-triazine, 1,1,3,3-tetramethylbutylamine polymer; (a PDYL with two polysubstituted piperidyl groups) showed embrittlement at 3085 hr. The combination of titanium dioxide pigment and the PDYL was not tested; nor was the addition of the 3,5-DHBZ to the foregoing combination. There was no suggestion that the addition of a PDYL to the 3,5-DHBZ in thin pigmented films (titanium dioxide, or any other) might produce different results, nor was there any reason to believe that their data, obtained by testing 200-300.mu. thick films, would not be applicable to films less than 50.mu. thick, or fibers.
More recently, a PDYL commercially available as Chimassorb 944, poly[6-[(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][2,2,6,6-tetr amethyl-4-piperidinyl)imino]]; has been combined with a 3,5-DHBZ and a phosphite commercially available as Ultranox (see Japanese publication JP-230401 (1987) to Sumitomo Chem Ind KK). But there is no indication that such a combination might have been notably effective to stabilize any pigment or dye.
It is also known that numerous pigments for PO, by themselves, provide a significant level of stabilization to PO, but there are many which have no noticeable effect on stability, and still others which accelerate degradation, that is, are prodegradants. To date, the only reliable method of determining to which group a pigment belongs, is by actually testing it in a particular substrate of interest. It is known, for example, a pigment which is a stabilizer in PP may be a prodegradant in a polyacetal.
Both phthalocyanine and azo pigments are known to have good color stability. They also provide some measure of light stability by virtue of their ability to block the path of radiation, thus shielding the pigmented polymer. Such small measure of light stabilization is observed in PP at about 0.4 phr. However, in combination with a stabilizing amount of a known PDYL primary stabilizer, which functions as a "radical trap" stabilizer, stabilization provided by 0.4 phr of an azo pigment is not substantial.
By a "primary" stabilizer, I refer to one which provides either long term thermoxidative stability during conditions to be encountered during use, or, UV light stability in bright, direct sunlight. Melt extrusion stability, especially for fibers of PO, is provided by a secondary stabilizer. Though the present invention does not require the use of any secondary stabilizer, in those instances, for example in fiber-spinning, where the melt is extruded at about 270.degree. C. (for PP fibers) more than once to obtain better pigment distribution, the PO may contain a small amount, no more than 0.1 phr, of a melt (or "process") stabilizer.
Typically, several additives are combined in PP before it is thermoformed, whether spun into fiber, injection molded, blow molded, extruded, etc., each additive being specifically designed to provide a different zone of stabilization, the main zones being (a) melt extrusion stability, (b) long term thermal stability during conditions expected to be encountered during use, (c) uv light stability in bright direct sunlight, and by no means of least importance, (d) stable tinctorial strength to maintain the desired color. Combining several additives known to be effective for each purpose, in PP articles is not likely to produce the desired results because of objectionable side effects due to interaction between the additives.
For example, thiodipropionate compounds such as dilauryl (DLTDP) and distearyl (DSTDP) help control melt-stability despite an odor problem, and certain phosphites control melt flow while depressing the tendency of PP fibers to yellow because the fibers usually contain a hindered phenol AO. The hindered phenol AO increases long term stability but accelerates yellowing. It is known that a hindered phenol AO and a thiodipropionate are most effective when used together. Certain PDYL HALS provide not only excellent uv stability but also such good long term thermal stability that the PP fibers will outlast some of the pigments used to color them. Yet such a HALS is typically combined with a hindered phenol and a phosphite.
Conventional wisdom dictates that if fibers stabilized with one or more stabilizers and a particular pigment meet the expectations of stability in the marketplace, then molded and extruded articles, other than fibers, will also be satisfactorily so stabilized. The opposite is not true. Therefore, pigments are selected with an eye to their effect upon the processing of PP fibers, the stability requirements of the end product, the pigment's interaction with the other additives to be used, the color requirements, and the cost of producing the pigmented PP fibers. The thrust towards using inexpensive PP fibers in the automobile industry where the colors red and bIue are in high demand, decreed that, despite their high cost, Red 144 and phthalocyanine blue be used, because of their intense tinctorial strength and color stability; and, that Red 144 and Blue 15 in particular, be combined with a compatible uv stabilizer. It was expected that one of the most damaging factors in the stability of Red 144-pigmented and Blue 15-pigmented PO fibers would be their interaction with the PDYL HALS used.
The commercial use of red and blue PP fibers requires that the color stability of the PP fibers be such that it equals the useful life of a fabric or other article containing the fiber, which article is exposed to heat and light. Because the stabilizers used generally affect color though they are not regarded as colorants, and, pigments affect thermal and uv light stability even if not known to have such activity, one cannot estimate what the net effect of the interactions might be. (see "Influence of Pigments on the Light Stability of Polymers: A Critical Review" by Peter P. Klemchuk, Polymer Photochemistry 3 pg 1-27, 1983).
I continued my tests with numerous combinations of stabilizers in Blue 15- and Red 144- and other azo-pigmented fibers, screening the samples to determine whether an unacceptable level of color loss was obtained before the fibers disintegrated. It was unimportant whether the combinations were of a primary with a secondary stabilizer, or, of co-primary stabilizers. The effectiveness of each combination was measured by the degree of degradation of the pigmented fibers both by visual observation, and by "scratch testing" (described herebelow) the surface of exposed fibers.
Fiber degradation is a phenomenon which is easily visible to the naked eye upon inspection of a degrading pigmented yarn exposed either in a Weather-O-Meter in the presence of moisture, or, to bright sun (tests are conducted in the Florida sun) under ambient conditions of humidity. Unstabilized Blue 15- or Red-144 pigmented PP fibers exposed to the Florida sun show no fading because the pigmented fibers degrade far more rapidly than the pigment, which results in continual sloughing off of layers of fiber, exposing bright undegraded pigment. Degradation of stabilized fiber is characterized by (i) a fuzzy, peach-skin-like appearance of the surface of the fabric (made with the pigmented fibers), and (ii) the problem of fading color.
For the simple reason that a large volume of PP goods are either extruded or molded, one way or the other, there was an urgent need to find an effective PDYL HALS which would provide such articles, as well as pigmented fibers, with adequate longevity under light-degrading conditions. To this end I searched for the appropriate HALS and for a compatible and effective co-stabilizer or "synergist" which might, in combination, provide the desired stabilization. Since there was no indication whether such a synergist should be, or might likely be either an AO or a uv-absorber, the search had to consider both.
As one would expect, some pigments enhance heat and light stability of PP fibers stabilized with a particular AO or uv-absorber. Other pigments have the opposite effect. Until tested, one cannot predict with reasonable certainty, what the effect will be. For example, with a nickel-containing stabilizer, Red 101 (iron oxide) is a prodegradant. With the more effective HALS, both Yellow 93 and Red 144 are prodegradants. The effect of these pigments in stabilized PP fibers could not have been predicted by their behavior in unstabilized pigmented fibers, or by their behavior with a different stabilizer.
With a nickel-containing stabilizer, Red 144 (unlike Red 101) is a stabilizer (not a prodegradant), but Red 144 is a prodegradant with Tinuvin 770. Yellow 93, a stabilizer when no other stabilizer is present, is neutral with nickel stabilization but is a prodegradant with Tinuvin 770 (see "Stabilization of Polypropylene Fibers" by Marvin Wishman of Phillips Fibers Corporation). Specifically with respect to red PP fibers, the problem was to find a combination of stabilizers which circumvented the proclivity of Red 144 to degrade the PP fibers and plaques when the pigment is combined with a conventional AO and uv light stabilizer. Because Red 144 was a prodegradant, it seemed desirable to use only as much of it as would provide the desired tinctorial effect for the required period of time, namely the useful life of the stabilized fiber.
The effect of a large number of pigments on the stability of PP fibers stabilized with Tinuvin 770 has been reported by Steinlein and Saar (see "Influence of Pigments on the Degradation of Polypropylene Fibers on Exposure to Light and Weather", paper presented at the 19th International Manmade Fiber Conference, Sept. 1980 in Austria).
In the same vein, like other workers before me, I tested a large number of combinations of primary stabilizers with Blue 15 and Red 144, both in fibers and in plaques.
The chemical peculiarity about an effective PDYL-based HALS is that it contains multiple polysubstituted piperidyl rings in a single molecule which is sometimes an oligomer. The most preferred of such piperidyl-based HALS are those which contain at least one triazine ring, and at least one substitutable position on each triazine ring is linked to a polysubstituted piperidyl ring.
Chimassorb 944 and Cyasorb UV 3346 poly[[6-(morpholinyl)-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-pipe ridinyl)imino]-1,6-hexanediyl[(2,2,6,6-tetramethyl-4-piperidinyl)imino]]; are HALS of relatively large size containing a polysubstituted hindered piperidyl ring and a triazine ring. Each HALS is an oligomer in which the repeating unit combines a hexamethylene diamine having polysubstituted piperidyl substituents on the N atoms, the substituted diamine unit being connected to a triazine ring in which one of the other substituents is either tert-octylamine (Chimasssorb 944), or, morpholone (Cyasorb 3346), and the other substituent is a hexamethylene diamine unit.
The unexpected and particularly noteworthy boost of color-stability derived from a 3,5-DHBZ, is thought to be due to the electron-withdrawing effect of the para-position of the ester substituent, but the highly surprising effect when the ester group is aryl, for example, 2,4-di-t-butyl is thought to be attributable to the photo-Fries rearrangement (when the 3,5-DHBZ is exposed to actinic radiation) which rearrangement can occur only with the aryl ester substituent. Other esters, particularly those derived from alicyclic or long chain aliphatic alcohols are comparably effective.
Prior to the publication of some of the studies which set forth a framework within which the foregoing factors are to be considered, a manufacturer of the 3,5-DHBZ supplied a sales flyer in which its UV-Chek AM-340 (2,4-di-tert-butyl phenyl ester of 3,5-di-tert-butyl-4-hydroxy-benzoic acid) stabilizer was stated to be "cost effective or synergistic with hindered amine light stabilizers, as well as, other stabilizer types, e.g., nickel organics, benzophenones, benzotriazoles, etc. in various polymers." The flyer further stated "In polymers, particularly polyolefins, UV-Chek AM-340 is a highly effective ultraviolet light stabilizer. It is especially effective in extruded and blown films, fine fibers, and molded articles. AM-340 is a white crystalline powder, and has no effect on the initial color of the polymer it is added to, and no effect on changing the shade of pigmented formulations. AM-340 is particularly useful in stabilizing pigmented polymers, especially in cases where the pigment itself contributes to the degradation of the polymer."
To the extent that such all-encompassing benefits of using AM-340 were not sales-oriented, they were based on data obtained with oriented PP film in 1.times.100 mil and 2.times.100 mil samples, and on 20 mil HDPE and 4 mil LLDPE plaques pigmented with 0.5% titanium dioxide. No observed color changes are provided, nor is there any indication that they were measured and found to show no change. As pointed out hereinabove, from the data presented by Leu, there was every reason to believe that such improvement as might be obtained by combining a piperidyl-based HALS with a 3,5-DHBZ would be additive at best when used in PO which is less than 50 microns thick. Further, the desirability of pigmented fibers was well known since before the time of publication of the sales flyer and, the lack of data on fibers is conspicuous by its absence.